Flat panel displays are now generally preferred over cathode ray tubes in a wide variety of applications, ranging from desktop and laptop computers to television screens. The perceived quality of an image reproduced by, say, a Liquid Crystal Display (LCD) depends greatly on its ability to provide high brightness and contrast. Such displays commonly use a backlight arrangement, where a light-diffusing plate is placed between a light source and the liquid crystal panel. It is also common for one or more collimating film sheets to be placed on or near the light diffusing plate. The collimating films serve the purpose of increasing the brightness of the display, a most desirable characteristic.
One type of collimating film incorporates a linear array of prisms molded or cast onto one side of film substrate. Typically the prisms have an included angle of 90°, and by the processes of refraction and TIR (Total Internal Reflection) recycles the on-axis light, and redirects the off-axis, or oblique, light into an on-axis emission direction. Furthermore, this type of collimating film is relatively easy produce, as both the molding process and the mold-generation (i.e., tooling) processes are relatively simple. Lastly, because the TIR and refracting processes are lossless (i.e., non-absorptive), this type of collimating film tends to have high lumen efficiency. However, this prism-based collimating film has two significant drawbacks. Firstly, the emission profile of the collimated light is fairly broad, with half angles of the emission envelope on the order of ±30°, which is generally much wider than the ideal envelope width for most applications. Secondly, the shape of the emission envelope can not be tailored or modified to suit the desired shape for the intended backlight application. That is, because the geometry of the 90° prisms is fixed for maximized collimation, there are no free parameters of the geometry that can be adjusted to modify the shape of the envelope.
Another type of collimating film incorporates an array of microlenses on one side of a substrate, with a reflective surface on the other side in which holes or apertures have been placed in correspondence with the microlenses. The reflective surface is placed towards the backlight or light source, and light from the source either reflects from the reflective surface or passes through an aperture. Nominally when light passes through an aperture it then is refracted by the corresponding lenslet into a collimated direction. There are several benefits of this type of collimating film over the prism-based collimating film. Specifically, the width of the emission envelope can be much less than ±30°, down to ±5° or less. Furthermore, because the amount of light condensation occurring is greater due to the reduced emission angles, the resulting luminance of the backlight that uses a microlens-based collimating element can be substantially improved.
However, for all its benefits, the microlens-based collimating film has proven to be difficult to manufacture. Typically the pitch of the microlenses is on the order of 50 μm, and the width of the apertures is on the order of 15 μm, and the apertures have to be well-aligned, or centered, with the optical axis of the lenslets in order to function. It has been found that producing an aperture that is a few microns in diameter in a reflective layer is problematic, but then doing so while at the same time achieving alignment with a lenslet is extremely difficult, especially over large areas.
Nonetheless, one way to overcome at least some of these difficulties is with a photolithographic process. According to this multi-step process, as taught in U.S. Pat. No. 6,633,351, the usual photolithography steps of 1) applying a resist (to the side opposite the microlenses), 2) exposure (through the microlenses), 3) development of the exposed resist, 4) etching, 5) resist removal, and then finally 6) reflective film formation. Furthermore, steps 3, 4, and 5 involve wet processing, and consume both substantial time and materials to complete. Therefore, the resulting collimating film will be exceptionally expensive to produce, and the end product will be prohibitively expensive to incorporate into any cost-sensitive backlighting application.
For further information on these types of films and backlit display systems see U.S. Pat. Nos. 5,453,876, 5,598,281, 5,870,224, 6,327,091, 6,421,103, 6,633,351, 6,697,042, and 6,876,408.